Alumina coating on piston

[divanis]

Hi everyone,

Not only recently, but for years we're having problems with hydraulic pistons used in petrochemical industry. Previously we used hard chromium and the result was "satisfying" (they need to change piston after 90 days of work, thus 4 pistons per year).
Also we tried flame spraying and fusing, but the result was pretty much the same.
Working conditions are: temp. 40 C, pressure 140 bar(>2000 psi), environment - solution of NH3 acid.
Lastly we tried to flame spray a Castolin al2O3+6% TiO2. We thought that alumina could resist the ammonia acid, and also provide good wear properties. Surface was prepared by sand blasting, preheated to 120 C, sprayed, grinded, polished and sealed with Loctite 290. The result was the coating "cracked" after 3 working days.(picture in attach.) Let me say that we used same material+technique on turbine journal (temp 450 C, pressure 20 bar, same environment) and it's working fine
We thought these cracks occurred due to some reaction between Loctite and ammonia acid, or maybe it could be due to higher porosity of flame sprayed coating, or flame spray cannot provide required mechanical properties (we also think of HVOF , but what kind of material).

We appreciate all your comments.

Kind regards to everyone.

[djewell]

You have localized cracking. To me, it looks like damage to the coating, not coating failure. There is no pitting or cracking evident in the rest of the picture, or am I missing something? My second thought would be that something contaminated the surface before spraying. Perhaps it was a lifting strap. I can see a rough surface in the coating hole, but that does not mean that there was no oil on the part before you started coating it.

Your speculation about porosity is interesting. High pressure oil could penetrate the coating and pop it off the substrate if it was not strongly bonded.

[Gordon]

Hi divanis

I reason that if a good fused coating is not providing the answer, then you need to try a fused coating containing tungsten carbide particles to beef up wear resistance. A HVOF/HVAF coating of WC/CoCr would be worth consideration.

A ceramic powder flame spray coating I think is just not up to this job. Maybe a plasma sprayed chromium oxide based ceramic on an (ammonia hydroxide? resistant) bond/base coat.

What bond/base coat were you using if any?

[divanis]

Hi divanis

I reason that if a good fused coating is not providing the answer, then you need to try a fused coating containing tungsten carbide particles to beef up wear resistance. A HVOF/HVAF coating of WC/CoCr would be worth consideration.

A ceramic powder flame spray coating I think is just not up to this job. Maybe a plasma sprayed chromium oxide based ceramic on an (ammonia hydroxide? resistant) bond/base coat.

What bond/base coat were you using if any?

Gordon, we were using Ni-Al bond coat. We believe that due to greater porosity of flame sprayed ceramic, ammonia solution penetrate through the bond coat and put it off.
Anyway, we opted for HVOF WC-Co-Cr, and PTFE sealant. I think it should be fine

Thanks for advice

Regards,

[MichiganMan]

Interesting photograph. Hypothetically, if this "cracking" occurred in approximately the sealing interface of the piston, then I would suggest that porosity is infact the culprit here. It looks almost as though pressure differential from one side of the seal to the other caused the hyraulic fluid to push through the coating (perhaps even through the loctite) and pop off coating on its way through. Perhaps it is just the light play on the particular photo, but there seems to be a pronounced color change from one side of the "failure" to the other which indicates to me more wear on one side than the other and therefore this would be the sealing (or perhaps, but less likely bearing) interface of the piston. Higher velocity spray method (i.e. HVOF) and a tighter sealer that is chemically compatible should solve the issue.

[loriolo]

This coating failure, was caused by a non adeguate coating selection, In this case plasma spray coatings are not a good solution, because the working pressure is too high for a plasma coating, this kind of plunger usually are coated with HVOF system and WC/Co coating material.

[Stephen Booth]

My two cents on this is to first ask for more information on the environment, what is NH3 Acid?

In some petrochemical plants they have plunger or pistons called Carbomate Plungers, in a similar environment, and thick chrome plating worked well, but many Thermal Spray coatings did not.

Without many more details, I would guess that Spray Fused Nickel based NiCrBSi material, at 50-60 HRc at 500 micron thickness would be a reliable choice depending on the packing, and actual environment, and of course the physical restrictions of the substrate, and the component size.

[loriolo]

The ammonia is a basic element, why you said “acid”, with which media it is mixed?
Normally your coating failed because Aluminum, copper and nickel are destroyed by ammonia, so they are not indicated for your application, so WcCo will perform better as hard coating, maybe there is a better choice if you know the exactly composition of the corrosive media.